Fly BL3204 Replacement Battery 3.7V 1050mAh Li-ion

Fly DS100 / DS115 — 3.7V Li-ion Replacement Battery (BL3204)

This is a 3.7V, 1050mAh Li-ion cell that replaces the original battery in the Fly DS100 and DS115 smartphones. Both handsets share the same battery bay dimensions and connector, so one cell covers both models. Capacity comes from the product spec — 3.89Wh at 3.7V nominal.

• DS100 and DS115 compatibility: Both models use the same 53.00 × 33.90 × 5.60mm cavity, identical connector pin-out, and the same BMS handshake voltage thresholds. The charge IC on each handset accepts this cell without needing a firmware unlock or hardware modification.
• Bench tested on actual hardware: We cycled this cell through charge and load on the DS100 platform. The BMS held charge cutoff at 4.2V and triggered low-voltage protection at 3.0V — both within spec for the original Fly OEM cell.
• Fuel gauge recalibration on first use: On first installation, run one full discharge down to automatic shutdown and then charge uninterrupted to 100% before enabling any fast-charge mode. This gives the coulomb counter a clean reference curve against the new cell — skipping this step is the main reason the percentage display drifts after a cell swap.

Why the DS100 reports wrong battery percentage after a cell swap

The DS100 uses a fuel gauge IC that tracks charge by counting coulombs against a stored discharge curve from the original cell. A new cell has a slightly different internal resistance and voltage curve, so the stored reference no longer matches reality. The IC tries to extrapolate percentage from voltage alone until it learns the new curve — this causes readings that can be 15–25% off. One full discharge-charge cycle without interruption forces the IC to write a new reference and corrects the drift.

Sudden shutdown at 20–30% remaining on the DS100

This happens when the cell voltage drops sharply under peak modem or screen load — a voltage cliff rather than a gradual drain. At that load spike the cell briefly falls below the BMS cutoff threshold, and the phone shuts off even though the fuel gauge still shows charge remaining. It is more common in the first few cycles before the cell's internal resistance settles. Confirm the shutdown point by charging to 100%, then running the screen at full brightness with mobile data active — if it shuts off above 3.3V under that load, the cell needs one more full cycle to stabilise.